Diffusion - return 404 errors for bad URIs

Summary: Fixes T5646. Makes diffusion a much better user experience. Users now see a 404 exception page when they have a bad URI. Previously, they saw a developer-facing raw exception.

Test Plan: played around in diffusion a bunch. most of these changes were fairly mechanical at the end of the day.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5646

Differential Revision: https://secure.phabricator.com/D11299

src/applications/diffusion/controller/DiffusionLintController.php

@@ -6,9 +6,8 @@ final class DiffusionLintController extends DiffusionController {
     return true;
   }
 
-  public function processRequest() {
-    $request = $this->getRequest();
-    $user = $this->getRequest()->getUser();
+  protected function processDiffusionRequest(AphrontRequest $request) {
+    $user = $request->getUser();
     $drequest = $this->diffusionRequest;
 
     if ($request->getStr('lint') !== null) {